A sheet glass, what is called "safety sheet glass", is used for a window glass of an automobile and the like. "Safety sheet glass" generally refers to two types of glass, i.e., a tempered glass (air-cooling tempered glass) and a laminated glass. In addition, these types of sheet glass usually are subjected to a bending process due to design necessities.
A laminated glass is commonly used for a windshield of an automobile. The Japanese Law as well as many other countries stipulates that a laminated glass should be used in a passenger car. The window glass for a windshield is generally attached to the body frame with an adhesive or a gasket.
Use of the laminated glass as a side window glass of an automobile has been suggested to improve its security against break-in and its soundproofing properties.
The laminated glass is composed of a plurality of single glass sheets, generally having a thickness of about 1.5 to 3.2 mm, that have been annealed, so that it is relatively thin. Therefore, the glass sheet may be broken by an impact to the edge of the glass sheet during handling in the production of the laminated glass or the assembly of it into an automobile or the like.
For a side window glass or a rear window glass of automobiles, a tempered glass is widely used. Although the side window glass is fixed to the body frame in some cases, it is mostly used in the form of a slide mechanism that allows a vertical movement, a hinged revolving mechanism, or a double-sliding mechanism using a rail, for the purpose of opening the window. In some types of automobiles, a glass sheet that moves up and down by a slide mechanism or a glass sheet hatch using a hinge is used for the rear window glass as well.
In particular, for the side window glass for a hinged door, the door may be opened and closed with the window glass partly open. In this case, the window glass is not entirely supported by the window frame, so that the window glass is susceptible to impacts at the time of opening and, especially, closing the door. The front door glass is particularly susceptible to impacts, because it is large and the front door is more frequently opened and closed. The front door glass in a hard-top car with no window frame is even more susceptible to impacts.
The above-mentioned (air-cooled) tempered glass poses no problem concerning its strength. However, when a laminated glass is to be used for a side window glass or the like, strength is a factor that has to be considered.
On the other hand, light weight has been increasingly demanded for parts in view of the fuel efficiency of automobiles. Single thin glass sheets are attached to each other to form a laminated glass for use as a windshield. In this case, as described above, although the glass sheet adheres to the body frame, again, strength is a factor that has to be considered.
It is possible to first obtain single glass sheets that are completely tempered and then to form a laminated glass using them (so-called "tempered-and-laminated glass"). In this case, the problem concerning strength is eliminated. However, if by any chance a tempered-and-laminated glass suffers stress of such magnitude that it cannot withstand, the tempered-and-laminated glass that has broken entirely to be divided into small pieces causes the problem that the visibility for the passengers is lost. Furthermore, when a glass sheet is thin, it may be impossible to temper it enough.
Therefore, in order to overcome the problem of losing visibility and to form a laminated glass with no problem concerning strength, it has been proposed to use semi-tempered single glass sheets. In this case, the problem concerning strength is not eliminated perfectly, although the problem of losing visibility when the sheet is fractured can be eliminated.
In this specification, so-called "semi-tempered glass sheets" are referred to as heat-strengthened glass sheets. A heat-strengthened glass sheet is a glass sheet obtained by subjecting a glass sheet to a heat treatment to form a compressive stress layer of a suitable size in the surface of the glass sheet, so that the breaking strength increases, and that the way it breaks is similar to that of the non-tempered glass sheet. See Japanese Industrial Standard (JIS) R 3222-1990 for details, the disclosure of which is hereby incorporated by reference. However, the invention is not limited to the thickness of the glass sheet exemplified in JIS R 3222-1990.
Now, regarding the above-mentioned laminated glass or a curved sheet glass, a variety of glass sheets have been proposed and produced. For example, a technique disclosed in Japanese Laid-Open Patent Publication (Published Japanese translations of PCT international publication for patent application; "Tokuhyo-Hei") No. 8-506564 will be described below.
"An apparatus and a method for controlling stress in a laminated glass for automobiles" of Japanese Laid-Open Patent Publication (Tokuhyo-Hei) No. 8-506564 disclosed a glass sheet "having an edge compression with the maximum at an edge of a glass sheet and decreasing from the edge toward an inner portion of the glass sheet, and having a net tension value and the maximum tension value at the inner peripheral portion (of the glass sheet)". Furthermore, in the glass sheet, "the maximum of the edge compression is larger than 300 kg/cm.sup.2 " and "the tension value is smaller than 60 kg/cm.sup.2 ".
The method for producing such a glass sheet includes the steps of "supporting a glass sheet on an annealing ring, cooling the glass sheet on the annealing ring so as to generate a permanent stress on the glass sheet, and insulating a peripheral portion of the glass sheet during cooling so as to maintain a temperature of the peripheral portion at a higher temperature than a strain point when the stress becomes permanent in the glass sheet" (see FIG. 7 in this specification).
According to FIG. 4 of this publication (FIG. 8 of this specification), in the distribution of the stress of the glass sheet according to the invention of Tokuhyo-Hei No. 8-506564, the compressive stress changes into tensile stress when moving away from the edge of the glass sheet toward an inner portion, and the tensile stress has a peak around 30 mm to 40 mm away from the edge. Thereafter, the stress becomes zero.
In other words, Japanese Laid-Open Patent Publication (Tokuhyo-Hei) No. 8-506564 shows the stress distribution of single glass sheets constituting a laminated glass in which the plane residual tensile stress has a peak.
Furthermore, as a result, according to the above-mentioned stress distribution of single glass sheets constituting a laminated glass, an annular plane residual tensile stress region is present in an inner portion along the peripheral portion of the glass sheet. The method for producing the glass sheet is a method utilizing a cooling method using an annealing ring.
Japanese Laid-Open Patent Publication (Tokkai-Hei) No. 4-231361 discloses "a window glass for automobiles" (see FIG. 9 of this specification). The window glass for automobiles disclosed in this publication is "produced from a complex glass sheet 201 formed of two thermally tempered single glass sheets 202 and 203 that are interconnected using a thermoplastic intermediate layer 204, particularly intended for a side window glass that is adjustable vertically, each of the two glass sheets having a thickness of 2.0 to 3.0 mm", "wherein a tensile stress at the center is between 54 and 76 MN/m.sup.2 in an edge region 205 of the glass sheet when the thickness of the single glass sheet is 2.0 mm, . . . a tensile stress at the center is between 38 and 60 MN/m.sup.2 in a central region 206 of the glass sheet when the thickness of the single glass sheet is 2 mm".
Japanese Laid-Open Patent Publication (Tokkai-Hei) No. 4-231361 refers to the tensile stress at the center of the glass sheet (i.e., the residual stress of the central layer of the glass sheet) as described above, but it does not directly refer to a plane residual stress.
However, the tensile stress at the center of the glass sheet is described, and there are explanations that "an edge region that is tempered more strongly", "because the strength of the edge region increases" or "the tempering of the edge region of the window glass is greater" or the like. These explanations result in the inference about the plane residual stress distribution of the glass sheet according to Japanese Patent Application Publication (Tokkai-Hei) No. 4-231361 that the stress in "the edge region" and the stress in "a visible region" are different.
Furthermore, there is the teaching that "in order to generate a higher and desired tensile stress in the edge region, it is necessary to ensure a larger release of heat in the edge region of the single glass sheet".
In other words, it is inferred that in a cooling method for obtaining the single glass sheet disclosed in Tokkai-Hei No. 4-231361, the cooling condition is not uniform in the area including the peripheral portion and the central portion of the glass sheet.
As a result, in the stress distribution of the single glass sheets constituting the laminated glass, an annular plane residual tensile stress region called "the edge region" in an inner portion along the peripheral portion of the glass sheet is present.
Japanese Patent Publication (Tokko-Sho) No. 63-17777 disclosed the following technique as "a method for producing a laminated glass", specifically a method for producing a curved and laminated glass. The method includes the steps of "laying two glass sheets each having a thickness of 1.5 mm to 3.2 mm on top of each other on a bending member so as to shape accordingly", "cooling the glass sheets at an annealing temperature between 450.degree. C. to 550.degree. C. at a cooling rate of 90.degree. C./min. to 150/min, so as to generate a plane compressive stress of 250 kg/cm.sup.2 to 500 kg/cm.sup.2 in a region not more than 1.5 cm apart from the edge of the glass sheets". A laminated glass is constituted by the thus produced glass sheets.
Japanese Patent Publication (Tokko-Hei) No. 6-29148 discloses "a method for thermally treating a glass sheet" comprising the steps of supporting "an edge peripheral portion of a glass sheet to be heated by a receiving ring frame" to be subjected to annealing, so as to generate a plane compressive stress of 50 kg/cm.sup.2 to 500 kg/cm.sup.2 in the edge peripheral portion of the glass sheet.